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Customize

docs/sandboxes/customize.mdx

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Before a sandbox starts doing real work, you can prepare it three ways. Scripts bundle reusable commands. Patches modify the rootfs before the VM boots. A custom init system (systemd, OpenRC, s6) can run as PID 1 instead of microsandbox's minimal agent. All three are defined at creation time and keep the base image untouched.

Scripts

Scripts are files mounted at /.msb/scripts/ inside the sandbox. The directory is on PATH, so each script is callable by name through exec() or shell().

It provides a clean way to bundle setup procedures or entry points with a sandbox without baking them into the image.

<CodeGroup> ```rust Rust use indoc::indoc; use microsandbox::Sandbox;

let sb = Sandbox::builder("worker") .image("ubuntu") .script("setup", indoc! {" #!/bin/bash apt-get update && apt-get install -y python3 curl "}) .script("start", indoc! {" #!/bin/bash exec python3 /app/main.py "}) .create() .await?;

sb.shell("setup").await?; let output = sb.shell("start").await?;


```typescript TypeScript
import { Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("worker")
    .image("ubuntu")
    .script("setup", "#!/bin/bash\napt-get update && apt-get install -y python3 curl")
    .script("run",   "#!/bin/bash\nexec python3 /app/main.py")
    .create();

await sb.shell("setup");
const output = await sb.shell("run");
python
from microsandbox import Sandbox

sb = await Sandbox.create(
    "worker",
    image="ubuntu",
    scripts={
        "setup": "#!/bin/bash\napt-get update && apt-get install -y python3 curl",
        "start": "#!/bin/bash\nexec python3 /app/main.py",
    },
)

await sb.shell("setup")
output = await sb.shell("start")
</CodeGroup>

Patches

Patches modify the rootfs before the VM boots. Write config files, copy directories from the host, create symlinks, append to existing files, remove things you don't need. The base image stays untouched since patches are written to the writable layer on top.

Patches are applied in order and work with OCI images and bind-mounted rootfs. They're not supported with disk image roots (QCOW2, Raw).

<Tip> By default, patching a path that already exists in the image will error. Pass `replace: true` on the operation to allow it. `Mkdir` and `Remove` are idempotent and won't error either way. </Tip> <CodeGroup> ```rust Rust use microsandbox::Sandbox;

let sb = Sandbox::builder("worker") .image("alpine") .patch(|p| p .text("/etc/greeting.txt", "Hello from a patched rootfs!\n", None, false) .text("/etc/motd", "Custom message of the day.\n", None, true) // replace existing .mkdir("/app", Some(0o755)) .text("/app/config.json", r#"{"debug": true}"#, Some(0o644), false) .copy_file("./cert.pem", "/etc/ssl/cert.pem", None, false) .append("/etc/hosts", "127.0.0.1 myapp.local\n") ) .create() .await?;


```typescript TypeScript
import { Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("worker")
    .image("alpine")
    .patch((p) => p
        .text("/etc/greeting.txt", "Hello from a patched rootfs!\n")
        .text("/etc/motd", "Custom message of the day.\n", { replace: true })
        .mkdir("/app", { mode: 0o755 })
        .text("/app/config.json", '{"debug": true}', { mode: 0o644 })
        .copyFile("./cert.pem", "/etc/ssl/cert.pem")
        .append("/etc/hosts", "127.0.0.1 myapp.local\n"),
    )
    .create();
python
from microsandbox import Patch, Sandbox

sb = await Sandbox.create(
    "worker",
    image="alpine",
    patches=[
        Patch.text("/etc/greeting.txt", "Hello from a patched rootfs!\n"),
        Patch.text("/etc/motd", "Custom message of the day.\n", replace=True),
        Patch.mkdir("/app", mode=0o755),
        Patch.text("/app/config.json", '{"debug": true}', mode=0o644),
        Patch.copy_file("./cert.pem", "/etc/ssl/cert.pem"),
        Patch.append("/etc/hosts", "127.0.0.1 myapp.local\n"),
    ],
)
</CodeGroup>

Available operations

The patch builder appends operations in the order you call them; calls are chainable. Available operations across SDKs: text, file, mkdir, append, copyFile / copy_file, copyDir / copy_dir, symlink, remove.

Full per-language signatures, parameters, and option shapes are documented in the SDK references:

Custom init system

By default the microsandbox agent runs as PID 1 inside the guest: small, fast, minimal. For workloads that expect a real init (systemd, OpenRC, s6, runit, etc.), --init hands PID 1 over to the init binary of your choice.

The handoff sequence:

  • Agent does the boot-time setup: mount filesystems, configure network, prepare runtime dirs.
  • Agent forks.
  • Parent execs your init and becomes PID 1.
  • Child agent continues serving host requests over the same channel.

Common reasons to opt in: long-lived daemons, system service tests, anything that talks to dbus or expects systemctl to work.

The simplest entry point is auto, which probes a small list of well-known paths inside the guest rootfs and picks the first one that exists:

  • /sbin/init
  • /lib/systemd/systemd
  • /usr/lib/systemd/systemd

Pass an absolute path instead when you need pinning (e.g. for reproducible CI).

<CodeGroup> ```rust Rust use microsandbox::Sandbox;

let sb = Sandbox::builder("worker") .image("ghcr.io/superradcompany/debian-systemd:12") .memory(1024) .cpus(2) .init("auto") .create() .await?;


```typescript TypeScript
import { MiB, Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("worker")
    .image("ghcr.io/superradcompany/debian-systemd:12")
    .memory(MiB(1024))
    .cpus(2)
    .init("auto")
    .create();
python
from microsandbox import Sandbox

sb = await Sandbox.create(
    "worker",
    image="ghcr.io/superradcompany/debian-systemd:12",
    memory=1024,
    cpus=2,
    init="auto",
)
bash
msb run ghcr.io/superradcompany/debian-systemd:12 \
  -m 1G -c 2 \
  --init auto \
  -- bash
</CodeGroup>

To verify the handoff worked, check /proc/1/comm inside the sandbox. It should print the init's name (systemd, init, etc.):

bash
$ msb run ghcr.io/superradcompany/debian-systemd:12 --init auto -- cat /proc/1/comm
systemd

If --init=auto can't find anything in its candidate list, agentd fails boot with a clear error in kernel.log listing every path it checked. Switch to an explicit path (--init=/lib/systemd/systemd) when you know exactly where the init lives, or follow the image-picking guidance below to choose an image that actually ships one.

Argv and env

Pass extra argv and env to the init:

  • Rust / TypeScript: init_with(...) / initWith(...).
  • Python: pass an InitConfig to init=.
  • CLI: repeat --init-arg once per entry, and --init-env KEY=VAL once per env var.

Argv defaults to [<cmd>] when none is given; env is merged on top of the inherited environment.

<CodeGroup> ```bash CLI msb run ghcr.io/superradcompany/debian-systemd:12 \ --init /lib/systemd/systemd \ --init-arg --unit=multi-user.target \ --init-env container=microsandbox \ -- bash ```
rust
let sb = Sandbox::builder("worker")
    .image("ghcr.io/superradcompany/debian-systemd:12")
    .init_with("/lib/systemd/systemd", |i| i
        .args(["--unit=multi-user.target"])
        .env("container", "microsandbox"))
    .create()
    .await?;
typescript
await using sb = await Sandbox.builder("worker")
    .image("ghcr.io/superradcompany/debian-systemd:12")
    .initWith("/lib/systemd/systemd", (i) => i
        .args(["--unit=multi-user.target"])
        .env("container", "microsandbox"))
    .create();
python
from microsandbox import InitConfig, Sandbox

sb = await Sandbox.create(
    "worker",
    image="ghcr.io/superradcompany/debian-systemd:12",
    init=InitConfig(
        cmd="/lib/systemd/systemd",
        args=("--unit=multi-user.target",),
        env={"container": "microsandbox"},
    ),
)
</CodeGroup>

Picking an image

Most slim Docker base images (debian:bookworm-slim, ubuntu:24.04, python:3.12-slim) are stripped of init binaries; they're built for "one process per container" and don't ship systemd at all. If you point --init at a path the image doesn't contain, the agent's pre-flight check fails boot with a clear error in the kernel log (no kernel panic).

Three ways to get an image with an init:

<AccordionGroup> <Accordion title="Use one of our published guest images"> We maintain a small collection at [`superradcompany/guest-images`](https://github.com/superradcompany/guest-images) for the cases where you specifically need a real init. Current set: 
| Image | Pull |
|-------|------|
| Debian + systemd | `ghcr.io/superradcompany/debian-systemd:12` |
| Ubuntu + systemd | `ghcr.io/superradcompany/ubuntu-systemd:24.04` |
| Fedora + systemd | `ghcr.io/superradcompany/fedora-systemd:40` |
| Alpine + OpenRC | `ghcr.io/superradcompany/alpine-openrc:3.20` |

Multi-arch (`linux/amd64` + `linux/arm64`), rebuilt weekly so they pick up upstream security patches, smoke-tested on every rebuild. Each image's GHCR page documents its specific tag aliases and contents.
</Accordion> <Accordion title="Build a small custom image"> ```dockerfile # Dockerfile.systemd FROM debian:bookworm RUN apt-get update \ && apt-get install -y --no-install-recommends systemd \ && rm -rf /var/lib/apt/lists/* ``` 
```bash
docker buildx build -t local-systemd:debian -f Dockerfile.systemd .
msb run local-systemd:debian --init /lib/systemd/systemd -- bash
```
</Accordion> <Accordion title="Use a community-built systemd image"> Examples: `jrei/systemd-debian:12`, `jrei/systemd-ubuntu:22.04`. These work out of the box but are published by community maintainers, not the distros themselves. Vet them like any other third-party image before running real workloads. </Accordion> </AccordionGroup>

For a sanity check that doesn't need systemd, alpine:3.20 ships BusyBox at /sbin/init, which is enough to exercise the handoff mechanics:

bash
msb run alpine:3.20 --init /sbin/init -- sh -c "cat /proc/1/comm"
# busybox

Shutdown semantics

Without --init, microsandbox shuts the guest down by remounting root read-only and calling reboot(RB_POWER_OFF). Typically <100 ms.

With --init, the agent isn't PID 1 anymore, so it asks your init to shut down:

  • SIGRTMIN+4 first (systemd's poweroff signal).
  • SIGTERM fallback for inits that don't speak it.

Your init then runs its own teardown (stop services, unmount, halt), which is slower:

InitTypical shutdown
BusyBox / s6 / runit<100 ms
OpenRC50-500 ms
systemd1-5 s

This is the price of a real init. If your test harness measures end-to-end sandbox lifecycle and you're comparing to a no-handoff baseline, account for this.

Init and entrypoint

--init and --entrypoint are orthogonal:

  • --init controls PID 1: what runs the system.
  • --entrypoint (and the trailing -- cmd) controls what your workload runs.

They can be combined. Boot systemd as PID 1 and have microsandbox exec calls land your shell, scripts, or app inside the systemd-managed environment.